Nonwoven products are widely used for filtration, absorption, adsorption, insulation, covering, supporting, and cushioning. For these applications, melt-blowing (MB) technology yields exceptional products, due to the ability to create very small fiber diameters and pores along with exceptionally large specific fiber surface area. In making such fibers, the melt-blowing apparatus is provided with an integral component known as the die tip or nosepiece, which filters the incoming polymer melt flow and converts it into a large number of liquid filaments of desired diameter for further treatment and bonding into a nonwoven web, substrate, or cartridge.
The die tip is a delicate and expensive device that requires trained and careful use and maintenance. Its features and condition directly affect production capacity and product quality. An exemplary die tip 1 is shown in FIG. 1. The die tip 1 attaches, via mounting screws 14, to a die body (not shown) having a die body channel into which a polymer melt or solution is introduced. For convenience herein, the polymer being extruded will be referred to as a “polymer melt” or “molten polymer”, but it should be understood that a polymer solution might instead be used.
The die tip 1 includes a centrally located channel 12 for receiving the polymer melt or solution. A filter media may be located in a recess 11 between the die body channel and the die tip channel 12 to prevent impurities from passing into the die tip channel 12. A row of orifices 13 is drilled through the downstream end of the die tip channel 12. The size of the orifices 13 is an important determinant to the average fiber diameter of the fibers produced thereby. The molten polymer stream exiting from the orifices 13 is attenuated by air knives of high pressure (not shown) to form fibers that are bonded on a collector surface, such as a conveyor belt or rotating mandrel, to produce a nonwoven substrate or cartridge.
There are several issues with existing die tips for melt-blowing apparatuses. First, die tip problems are common and costly to the manufacturing process. Routine wear, aging, and contamination may cause costly product irregularities or off-quality. In the most extreme cases, the die tip may be cracked open by a pressure surge from the polymer melt (a phenomenon known by operators as a “zipper” or “unzipper” condition). As shown in FIG. 2, the die tip 1 has a “wishbone” shape 2 that is susceptible to pressure forces in a lateral direction 21, which may lead to cracking in an area 22 slightly upstream of the orifice 13.
Occasionally, manufacturers try to repair the die tip 1 for continued use. The use of repaired die tips is risky, especially in heavy-duty environments. However, such risks must be weighed against the time and expense associated with obtaining replacement die tips or with maintaining an inventory of spare die tips. Another issue is that various die tip fabricators build die tips with different design details, so the die tips generally are not interchangeable among apparatuses.
Since the introduction of melt blowing technology, engineers have made repeated efforts to improve the original die tip design (as shown in FIG. 1). Representative patents in this area include, without limitation:
A. U.S. Pat. Nos. 3,825,379; 5,017,112; and 5,171,512 suggest the use of capillary tubes to replace drilled orifices in the die tip. As shown in FIG. 3, a die tip 3 includes a die tip apex 32 through which are arranged a row of capillary tubes 31, which convey the polymer melt into the attenuating air knives. The improvement in fiber extruding capability is not convincing. Moreover, because the die tip apex 32 is separated to accommodate the capillary tubes 31, the die tip body is weakened. As shown in FIG. 2, the weakened die tip body is prone to cracking. Thus, with no convincing evidence of improvement in fiber extrusion capability, the use of this particular design appears without merit.
B. U.S. Pat. No. 3,849,241 teaches a split-body die tip, in an effort to avoid the tedium and expense of the orifice drilling procedure. As mentioned above, a split die tip body is structurally weak and exhibits low resistance to internal fluid pressure. While this approach may be useful on small laboratory models, it lacks the robustness and integrity required for production scale.
C. U.S. Pat. No. 3,865,535 attempts to provide a split-body die tip for production scale, that is, for wide melt-blowing apparatuses. As shown in FIG. 4, a die tip 4 is made of two halves joinable along the longitudinal axis of the die tip 4. A side cross-sectional view, as taken along line A-A of the perspective view, shows a melt reservoir 42, which delivers polymer to a row of drilled orifices 41. The two dip tip halves are assembled together by bonding agents applied on the available contact area 43, namely at the two ends and part of the apex area. Substances suggested for bonding the die tip halves include silver brazing solder, thermosetting epoxy resin, and other bonding agents for metals. In practice, such bonding agents were insufficient to hold the die tip together for a long time and under high internal pressure. Additionally, the routine cleaning of the die tip in an autoclave exposes the die tip to temperatures of 1,000° F. to 1,500° F. for hours, which reduces the life of the bonding agents. For these reasons, the bonded die tip fails to meet the needs of the industry.
D. U.S. Pat. No. 4,486,161 aims to improve the pressure resistance of a die tip 5 by employing several large screws 51 to hold together the upstream and downstream portions of the die tip body, as shown in FIG. 5. In this die tip, screws 51 are located at equal spacing along the length of the die tip body and are pre-tightened in anticipation of the melt pressure. As a consequence, a large and dangerous bending moment is applied to the die tip apex through which the orifices 53 are drilled. Another issue with this design is that, because large bolts (51) sit inside the die tip reservoir 52, the flow uniformity created by the upstream die body is disrupted and the maintenance of the die tip apex is complicated.
E. U.S. Pat. No. 4,986,743 suggests a technique to pre-stress a die tip 6, when it is installed onto the melt-blown die body 62, as shown in FIG. 6. Mounting screws 61 are used to secure the die tip body 6 to the die body 62. The die tip 6 includes drilled orifices 63 for producing melt-blown fibers. The die tip 6 is provided with gaps between the upstream surface of the die tip body and the downstream surface of the die body 62, creating a fulcrum point 64 about which the die tip body 6 is bent. The gaps and screws 61 induce a stress, which is purported to enhance the pressure tolerance of the die tip 6. While this idea is simple and may be valid, its implementation requires skill and precision, and its actual benefit is incremental and imprecise.
F. U.S. Pat. No. 4,720,252 proposes another concept for a split-body die tip 7, as shown in FIG. 7. In die tip 7, the polymer flows from a melt reservoir 71 through drilled orifices 72 in a die tip apex 73. Heated, pressurized air from air knives 74 flows through air gaps 75 to attenuate the melt-blown fibers. Within the air gap 75, multiple brace pieces 76 are spaced evenly and are welded to both the die tip apex 73 and the adjacent edges of the air plates 74 to buttress the halves of the die tip apex 73. The multiple brace pieces 76, which are small metal plates, compromise the uniformity of the air knives. Additionally, because the air plates 74 and the brace pieces 76 are relatively thin and are incapable of providing significant support, the integrity of the die tip 7 remains weak. Operation and maintenance of a melt-blowing apparatus using this die tip 7 are complicated, making the merit of this proposed device questionable at best.
As evident from the preceding discussion, none of the prior art inventions has successfully replaced the original and conventional design of the die tip, as shown in FIG. 1. Since none of the prior art inventions has successfully addressed the problems of the conventional die tip, a need for an improved die tip remains unmet in the industry.
Another shortcoming shared by the die tip assemblies of FIGS. 1 through 7 is their collective inability to produce fibers of a very small diameter (e.g., submicron size fibers and smaller). A micron is one-millionth of a meter. The term “nano-fibers” is used to describe fibers having an average diameter size measured in nanometers (nm), or one-billionth of a meter. The term “submicron fibers” refers to fibers having an average diameter size of between 500 and 999 nm. Nano-fibers having an average diameter of between 100 and 500 nm and submicron fibers having an average diameter of between 500 and 999 nm are of particular commercial interest and are difficult to obtain using presently available die tips.
Since the introduction of very small fibers, they have been used in numerous new applications, such as biomedical (e.g., synthetic tissues, organs, skin, blood vessels; wound healing; drug release; metal ion adsorption for detoxification), carriers for enzymes and catalysts, sensors, weapons and warfare, environmental protection, water/gas filtration and purification, personal protection/care, energy devices (e.g., lithium batteries, super capacitors, fast chargers, solar cells, fuel cells, hydrogen and natural gas storage/transportation, renewable energy harvest and storage, electric vehicles), electronics, membrane replacements, and the like.
In the production of very fine fibers, such as submicron and nano-fibers, melt-blowing competes with electro-spin technology. Electro-spin technology has successfully produced fibers as fine as 100 nm in tiny quantities, while the best available melt-blowing devices are capable of producing slightly coarser fibers ranging from 350 to 500 nm with greater economy. Accordingly, both technologies have a potential for advancement and would benefit from improvements thereto.
Recent studies and the inventor's experience suggest that the major impediments to creating melt-blown nano-fibers (less than 500 nm) are the currently available die and the die tip. Conventional dies and die tips used for extruding melt-blown fibers are unable to process melts of super low viscosity and are unable to withstand the extremely high pressures required for very small orifices. For example, conventional dies and die tips are typically designed for pressures of about 600 psi to 900 psi (pounds per square inch), whereas pressures of 3000 psi or greater may be necessary for producing very fine fibers. Conventional die tips, such as those described above, also simply do not have orifices that are small enough to produce the target fiber size. Specifically, to produce very fine (submicron or nano-) fibers successfully, the die tip requires orifices with smooth and uniform walls and with diameters in the range of 0.01 mm to 0.1 mm, while the smallest orifices in currently available die tips are only 0.2 mm (0.008 inches), and die tips having these tiny orifices are available only from a limited number of sources at often prohibitively high costs.
Raising the air-to-melt mass flow ratio and lowering the melt viscosity are known to help reduce fiber diameter. However, when the force of wind shear becomes too strong and the visco-elastic properties and surface tension of the melt become too weak for fiber forming, a spray of droplets results instead of melt filaments. Such a situation was observed by Ellison et al., in their article “Melt Blown Nanofibers: Fiber Diameter Distributions and Onset of Fiber Breakup”, published in 2007 by Elsevier in Volume 48, pages 3306-3316, of “Polymer” journal. The authors were able to produce melt-blown fibers only as fine as 350 to 500 nm on a laboratory scale apparatus.
Therefore, there has long been a strong desire for a better die tip design that is economical, simple, rugged, pressure resistant and that is easy to build, maintain, and repair. Moreover, there is a need in the industry for a die tip capable of withstanding high pressures in a mass production environment, which includes super small orifices for producing very fine fibers in the micron, submicron, and nano scales.